The present invention relates to power transistor devices, and particularly to short circuit and overcurrent protection circuits and methods for power transistor devices and in particular MOS gated power transistors. Even more particularly, the present invention relates to a method and apparatus for protecting insulated gate bipolar transistors under short circuit and overcurrent conditions.
A critical consideration in applying IGBTs to pulse width modulated (PWM) inverter AC motor drives is to protect them under short circuit conditions that can occur from line to line or line to ground.
IGBTs are available that have short circuit times approaching those of bipolar transistors. These types can be protected by the same relatively slow acting circuitry used for bipolar transistors.
The most efficient IGBTs, however, offer significant advantages in terms of lower losses, smaller packages, smaller heat sinks and compactness, but have lower short circuit times, i.e., they can tolerate short circuit conditions for a shorter period of time than bipolar transistors.
Traditional protection methods for bipolar transistors are inapplicable to these high efficiency IGBTs, because they do not provide the necessary speed in detecting a short circuit or over current condition. By the time that these traditional circuits respond, the IGBT may have been destroyed by the overcurrent.
IGBTs are becoming devices of choice in applications such as variable speed motor controllers, uninterrupted power supplies and high frequency welders. They generally offer comparable or lower power dissipation, higher operating frequency and simplification of drive circuitry. Thus, systems using IGBTs offer greater compactness, greater efficiency and superior dynamic performance.
The properties of the IGBT that make these advantages possible bring with them a new design consideration. An IGBT that is designed to achieve maximum efficiency has a relatively high gain and thus a short circuit current that is significantly higher than that obtained with a bipolar transistor. The power density in the IGBT with an applied short circuit can therefore be much higher than in the bipolar transistor.
An IGBT that is designed to minimize power dissipation under normal load conditions, therefore, may not be able to handle an unabated short circuit for as long as a bipolar transistor. It is not as fault tolerant, and therefore requires a more "alert" or fast acting protective circuit. The circuit of the invention provides a means which is particularly useful in protecting IGBTs against such short circuits.